Optical compensator and liquid crystal display

ABSTRACT

There is provided a laminate consisting of a first oriented film (1) obtainable by drawing, at least in one direction, a polymer film having a refractive index of n D  ≧1.60, an Abbe number of ν D  ≧30.0 and a glass transition temperature of Tg=60° to 160° C. and a second oriented film (2) obtainable by drawing, at least in one direction, a polymer film having a refractive index of n D  &lt;1.60. The laminate has a retardation value R of 60-1000 nm and a wavelength dispersion value ν RF  of not less than 1.10. This oriented film either as it is or as laminated with an optically isotropic film at least on one side is used to fabricate an optical compensator for color compensation in a liquid crystal panel. In this manner, there can be obtained a liquid crystal display panel which solves the long-standing problems of coloration and low contrast ratio.

TECHNICAL FIELD

The present invention relates to an optical compensator capable ofproviding a liquid crystal display device with improved displaycoloration and contrast ratio.

BACKGROUND ART

As a liquid crystal display device utilizing a supertwisted nematicstructure, an STN liquid crystal display of the construction: firstpolarizer/driver liquid crystal cell/hue compensating liquid crystalcell/second polarizer system has been developed [Nikkei Microdevices,August 1987, pp. 36-38 and Nikkei Microdevices, October 1987, pp.84-88].

The light incident on and passing through the first polarizer becomeslinear polarized light which, in turn, is converted to ellipticallypolarized light by birefringence as it passes through the driver liquidcrystal cell. The elliptic polarization rate and orientational angleinvolved are dependent on the wavelength. However, the light emergingfrom the driver liquid crystal cell is twisted in the reverse directionas it passes through the hue-compensating liquid crystal cell, so thatthe elliptically polarized light is reconverted to linearly polarizedlight (that is to say the phase difference is cancelled), which is takenout through the second polarizer. In this way, the wavelength dependenceof transmitted light is eliminated and a substantially white-and-blackdisplay is obtained. Therefore, if necessary, a full-color display canbe implemented by adding color filters.

The above STN liquid crystal display incorporating a driver liquidcrystal cell and a hue compensating liquid crystal cell is thick andheavy and, as an additional disadvantage, costly to manufacture. Therealso is the problem that the display is too dark in the reflecting mode.

Therefore, to overcome these disadvantages, a Formulated Super-TwistedNematic system (hereinafter referred to as FTN mode) incorporating anoptical compensator comprising a monoaxially oriented polymer filmlaminated with an optically isotropic film on either side thereof inlieu of said hue compensating liquid crystal cell is attractingattention. The basic architecture of this FTN liquid crystal display is:polarizer/liquid crystal cell/optical compensator/polarizer.

Japanese Patent Application Kokai No. 64-519, as filed earlier by one ofthe inventors of the present invention, discloses that, as themonoaxially oriented film mentioned above, polyvinyl alcohol, polyester,polyetheramide, polyethylene, etc. can be employed.

Japanese Patent Application Kokai No. 1-118805 describes an opticalcompensator obtainable by orienting a film of polyvinyl alcohol or aderivative thereof in one direction, treating the oriented film with anaqueous boric acid-containing solution and laminating an opticallynon-oriented poller film on one or either side of said oriented film.The derivative of polyvinyl alcohol mentioned above means apolyvinylacetal such as polyvinylbutyral, polyvinylformal, etc.

Japanese Patent Application Kokai No. 1-118819 and Japanese PatentApplication Kokai No. 1-124821 disclose the use of an opticallycompensating film comprising an oriented synthetic resin film or anoptical compensator comprising said optically compensating film and anoptically isotropic amorphous film laminated at least on one sidethereof as one of the transparent electrode-supporting substrates of aliquid crystal cell. Japanese Patent Application Kokai No. 1-127329discloses a laminate having an optical compensating function which isobtainable by laminating an optical compensator similar to the abovewith a release sheet through an adhesive layer. It is disclosed in thesepatent literature that polycarbonate, phenoxy resin, polyparabanic acidresin, fumaric acid resin, polyamino acid resin, polystyrene,polysulfone, polyether polysulfone, polyarylene ester, polyvinylalcohol, ethylene-vinyl alcohol copolymer, polyvinyl chloride,polymethyl methacrylate, polyester, cellulosic polymer, etc. can beemployed. Incidentally, it is to be noted that these patent applicationsas well as Japanese Patent Application Kokai No. 2-158701 referred tobelow are all those filed by another applicant among the presentapplicants.

Japanese Patent Application Kokai No. 2-158701 discloses a compositeoptical compensator comprising a birefringent multi-layer filmobtainable by laminating a plurality of low-oriented birefringent unitcast films having a retardation value of 30 to 1000 nm with alignment ofrespective optic axes and, as film materials, mentions crosslinkingresins such as phenoxyether crosslinking resin, epoxy resin, acrylicresin, urethane resin, etc., polycarbonate, polyarylene ester,polyethersulfone, polysulfone, polyethylene terephthalate, polybutyleneterephthalate, polyvinyl chloride, polystyrene, ethylene-vinyl alcoholcopolymer, polyvinyl alcohol, amorphous polyolefin, fumaric acid resin,polyamino acid resin, ABS resin and so on.

Japanese Patent Application Kokai No. 2-256003 discloses an opticalfilm, primarily intended for an optical compensator, which is obtainableby orienting a thermoplastic polymer film without thickness variationmonoaxially at right angles with the extruding direction or biaxiallyand having a retardation value of not more than 1200 nm with a varianceof not more than 10% in retardation value and, as said thermoplasticpolymer, mentions polycarbonate resin, poly(meth)acrylate resin,polystyrene resin, acrylonitrile resin, polyester resin (polyethyleneterephthalate, polyester copolymer, etc.), polyamide resin, polyvinylchloride, polyolefin resin, polysulfone, polyethersulfone, fluororesinand so on.

Japanese Patent Application Kokai No. 2-256023 discloses a liquidcrystal display including a film of planarly oriented molecules having anegative intrinsic birefringence value and a monoaxially oriented filmof a polymer having a positive birefringent value as interposed betweena liquid crystal cell and a polarizer, and mentions, as examples of theformer polymer, polystyrene and acrylate pollers and, as examples of thelatter polymer, polycarbonate, polyarylate, polyethylene terephthalate,polyethersulfone, polyphenylene sulfide, polyphenylene oxide,polyallylsulfone, polyamideimde, polyolefin, polyacrylnitrile, celluloseand polyester.

Japanese Patent Application Kokai No. 2-257103 teaches an opticalcompensator comprising a laminate of an optically compensating filmobtainable by monoaxial orientation of a polyvinyl alcohol film andhaving a retardation value of 300 to 800 nm with a polysulfone orpolyarylate film.

However, with any of the optically compensating films comprisingmonoaxially oriented films formed from the polymers described in theabove profusion of literature or any of the optical compensatorsfabricated by laminating an optically isotropic film on one or eitherside of said optically compensating film, irrespective of whether themonoaxially oriented optically compensating film is used in a singlelayer or in a plurality of layers, it is impossible to compensate forthe phase difference caused by the STN cell over the entire wavelengthregion, thus failing to fully solve the problems of coloration and lowcontrast ratio. Therefore, although these technologies are able to solvethe problems of great thickness and weight which are inevitable with theSTN liquid crystal display mode employing a driver liquid crystal celland a hue compensating liquid crystal cell, they are inferior to themode employing a hue compensating liquid crystal cell in coloration andcontrast ratio. This aspect is an important problem to be solved of theFTN mode employing an optical compensator made of polymer film.

The object of the present invention is to provide a radical solution tothe long-standing problems of coloration and low contrast ratio in theFTN mode employing an optically compensating film or an opticalcompensator for hue compensation in a liquid crystal display.

DISCLOSURE OF INVENTION

The optical compensator of the present invention is a laminate filmcomprising (1) a first oriented film obtainable by orienting, at leastin one direction, a polymer film having a refractive index of n_(D)≧1.60, an Abbe number of ν_(D) ≦30.0 and a glass transition temperatureof 60°-160° C. and (2) a second oriented film obtainable by orienting,at least in one direction, a polymer film having a refractive index ofn_(D) <1.60.

It is particularly preferable that the retardation value R of thelaminated film is 60-1000 nm and that the wavelength dispersion valueν_(RF), which is defined below, of the first oriented film (1) of thelaminate is not less than 1.10.

    ν.sub.RF =Δn·d (450 nm)/Δn·d (590 nm)

The present invention is now described in detail hereinafter.

First oriented film (1)

As mentioned above, the first oriented film (1) is an oriented filmwhich is obtainable by orienting, at least in one direction, a polymerfilm which satisfies all of the following requirements, namely

Refractive index n_(D) ≧1.60

Abbe number ν_(D) ≧30.0

Glass transition temperature T_(g) =60°-60° C.

The refractive index n_(D) stands for the refractive index relative tothe sodium D line (589 nm) as measured in accordance with ASTM D-542. Ifthe refractive index n_(D) is less than 1.60, the problems of colorationand low contrast ratio cannot be solved even if the other requirementsare met.

The Abbe number ν_(D) is an indicator expressed by the equation ν_(D)=(n_(D) -1)/(n_(F) -n_(C)), wherein n_(D), n_(F) and n_(c) arerefractive indices with respective to the D line (589 nm), F line (486nm) and C line (656 nm), respectively. If this Abbe number exceeds 30.0,the problems of coloration and low contrast ratio cannot be solved evenif the other requirements are met.

It is also necessary that the glass transition temperature T_(g) iswithin the range of 60° to 160° C. If the glass transition temperatureis below 60° C., heat resistance will be inadequate. On the other hand,drawability is scarified when the glass transition temperature exceeds160° C.

As polymers that may provide polymer films meeting all of the abovethree requirements, there can be mentioned, among others, brominated orchlorinated phenoxyether polymer, polyethylene naphthalate,bisphenol-aromatic dicarboxylic acid polycondensate,polyvinylnaphthalene, polyvinylcarbazole, polypentabromophenylmethacrylate, polypentachlorophenyl methacrylate, poly(α-naphthylmethacrylate), poly(p-divinylbenzene) and so on. What is essential isthat the film ultimately meet the above refractive index n_(D), Abbenumber ν_(D) and glass transition temperature T_(g) requirements; thatis to say it may be a copolymer (inclusive of graft copolymer) film, afilm of coexisting polymers, a post-modified polymer film, a polymerblend film composed of two or more polymers with dissimilarcharacteristic values, or a laminate of a plurality of polymer films.

Among the above polymers, brominated or chlorinated phenoxyethercrosslinking resin and polyethylene naphthalate are particularlyimportant. While the characteristic values of these polymer films aredependent on the molecular weight, film-forming technology and degree ofhalogenation, among other things, some typical characteristics valuesare shown below.

    ______________________________________                                                         n.sub.D                                                                              ν.sub.D                                                                           T.sub.g                                        ______________________________________                                        Brominated phenoxyether polymer                                                                  1.64     24     149                                        Chlorinated phenoxyether polymer                                                                 1.63     25     140                                        Polyethylene naphthalate                                                                         1.65     19     113                                        Polyvinylnaphthalene                                                                             1.68     21     158                                        Polyvinylcarbazole 1.68     19      84                                        Poly(p-divinylbenzene)                                                                           1.62     28     106                                        ______________________________________                                    

The polymer films heretofore proposed as phase difference films aredeviating from the above-mentioned range of refractive index n_(D), Abbenumber ν_(D) or glass transition temperature T_(g) and are, therefore,not effective enough to accomplish the objectives.

Polycarbonate: n_(D) =1.58-1.59

Polymethyl methacrylate: n_(D) =1.49, ν_(D) =57

Polyvinyl alcohol: n_(D) =1.49-1.53

Polyethylene terephthalate: n_(D) =1.53

Polyethylene: n_(D) =1.51

Polypropylene: n_(D) =1.49

Polyvinyl chloride: n_(D) =1.54-1.55

Polysulfone: T_(g) >160° C.

Polyethersulfone: T_(g) >160°

Polyarylate. n_(D) =1.61, ν_(D) =26, T_(g) =215° C.

Polystyrene: n_(D) =1.59, ν_(D) =31

Polyphenylene oxide: T_(g) =209° C.

Polyacrylonitrile: n_(D) =1.52, ν_(D) =52

Cellulosic polymer: n_(D) =1.49-1.51

Amorphous polyolefin: n_(D) =1.52

Nylon 66: n_(D) 1.52-1.53, ν_(D) =40

ABS resin: n_(D) =1.54

Polyester copolymer: n_(D) =1.52-1.57

Phenoxyether polymer, neither brominated nor chlorinated: n_(D) <1.60

Polytetrafluoroethylene: n_(D) =1.35

It is particularly desirable that said first oriented film (1) has awavelength dispersion value of ν_(RF) ≧1.10. The wavelength dispersionvalue ν_(RF) is an indicator which is defined by the following equation.

    ν.sub.RF =Δn·d (450 nm)/Δn·d (590 nm)

If this value is less than 1.10, the dispersibility will be inadequateso that the problems of coloration and low contrast ratio of the displaycannot be solved. The wavelength dispersion value ν_(RF) preferablyapproximates the value of the liquid crystal, namely 1.10 to 1.18.However, depending on compensating conditions for the liquid crystalcell, there are cases in which a marked color compensation effect can beobtained even at a large value of 1.2.

The research conducted by the inventors of the present invention led tothe finding that when a polymer film having a refractive index of n_(D)≧1.60, an Abbe number of ν_(D) ≦30.0 and a glass transition temperatureof 60°-160° C. is oriented in at least one direction, the resultingoriented film has an increased wavelength dispersion value ν_(RF).Therefore, it is important to select the proper thickness and drawingconditions for polymer film so that the wavelength dispersion valueν_(RF) of the first oriented film (1) will be not less than 1.10.

The polymer film having the above characteristic values can be obtainedby the casting or melt-forming technology. Drawing of polymer films isgenerally carried out at or around a temperature higher than the glasstransition temperature T_(g) by 5° to 40° C., particularly about 10° to30° C., and the drawing is preferably followed by aging. In many cases,the draw ratio is approximately 1.1 to 6, particularly 1.2 to 4, perdirection. It is also possible to draw film in one direction withrestriction of draw ratio in a perpendicular direction or restriction ofshrinkage in a perpendicular direction and, in such a case, the filmbecomes a biaxially oriented film.

Second oriented film (2)

The second oriented film (2) is an oriented film obtainable by drawing,at least in one direction, a polymer film having a refractive indexn_(D) value of less than 1.60. However, if the refractive index n_(D) isreduced too much, random reflection tends to occur at the interface withthe first oriented film (1). Therefore, the refractive index n_(D) ispreferably not less than 1.50.

The polymer film for the production of the second oriented film (2),unlike the polymer film for the production of the first oriented film(1), is not particularly limited in the aspect of Abbe number ν_(D) andmay be either less than 30.0 or more than 30.0. However, since few filmshave Abbe numbers less than 30.0, it is usual that the Abbe numberexceeds 30.0. This film is not limited in glass transition temperatureT_(g), either, but this characteristic value is preferably not higherthan 160° C. from the standpoint of ease of drawing. Since the heatresistance is compensated for by the first oriented film (1), the glasstransition temperature T_(g) may be lower than 60° C.

As examples of the polymer for this polymer film, there can be mentionedpolycarbonate, phenoxyether polymer, polystyrene, polyvinyl alcohol,polyvinyl chloride, polyvinylidene chloride, cellulose triacetate, epoxyresin, ABS resin and so on.

Laminate film

The optical compensator of the present invention comprises a firstoriented film (1) and a second oriented film (2). The laminar structureis optional and may for example be (1)/(2), (1)/(2)/(1), (1)/(1)/(2),(2)/(1)/(2)or (2)/(1)/(2)/(2).

It is particularly preferable that the retardation value R of thelaminate film be 60 to 1000 nm. If the retardation value R of thelaminate film is less than 60 nm, the phase difference function will beinadequate. On the other hand, when the retardation value R exceeds 1000nm, the film thickness must be markedly increased but if it be so done,the optional homogeneity is sacrificed and the hue compensating effectis reduced.

The retardation value R, referred to above, is an indicator which can beexpressed by the following equation.

    R=d·|n.sub.1 -n.sub.2 |=Δn·d

(wherein d represents the thickness of film, n₁ represents therefractive index in the direction of optic axis or the direction atright angles therewith, n₂ represents the refractive index in thedirection perpendicular to the direction of n₁, and the refractive indexis a value relative to the sodium D line).

The optical compensator may be constituted of such a laminate film alonebut an optically isotopic film (3) may be further laminated at least onone side of the laminate film for purposes of protection.

The optically isotopic film (3) for such purposes includes the films of,among others, cellulosic polymer (e.g. cellulose triacetate),polycarbonate, polyparabanic acid resin, polystyrene, polyethersulfone,polyarylene ester, polysulfone, polyvinyl chloride,poly-4-methylpentene, polyphenylene oxide, oxygenimpermeable resin,crosslinked resin and so on.

The oxygen-impermeable resin mentioned above includes polyvinyl alcohol,ethylene-vinyl alcohol copolymer, polyacrylonitrile, polyvinylidenechloride, etc. and the crosslinked resin includes crosslinkedphenoxyether resin, epoxy resin, acrylate resin, acrylepoxy resin,urethane resin, etc. Not only a single-layer film but also a compositefilm such as an ethylene-vinyl alcohol copolymer/phenoxyethercrosslinked resin film can be employed. The retardation value of theoptically isotropic film is preferably not greater than 30 nm and, forstill better results, not greater than 10 nm.

The ease of handling up to the fabrication of a liquid crystal displaycan be insured by laminating a release sheet (5) through apressure-sensitive adhesive layer (4) on at least one side of theoptical compensator of the present invention.

The optical compensator of the invention can be assembled with apolarizer to provide a optical compensator equipped with a polarizer orused as the substrate of a liquid crystal cell or laminated with aliquid crystal cell substrate prior to the fabrication of the liquidcrystal cell to provide a liquid crystal cell panel equipped with aoptical compensator.

Operation

As mentioned hereinbefore, it was discovered that when a polymer filmhaving a refractive index of n_(D) ≧1.60, an Abbe number of ν_(D) ≦30.0and a glass transition temperature T_(g) of 60°-160° C. is oriented inat least one direction, the wavelength dispersion value ν_(RF) isincreased as an "attribute" of such film. Therefore, the first orientedfilm (1) having a wavelength dispersion value of not less than 1.10 canbe obtained by selecting and drawing a polymer film having suchcharacteristic value. Since this first oriented film (1) has a largewavelength dispersion value ν_(RF), the use of this film as an opticallycompensating film results in marked improvements in coloration andcontrast ratio.

However, if this first oriented film (1) is used alone, it is impossibleto obtain an optical compensator having a retardation value in the rangeof 60 to 1000 nm, particularly in the optimum range of 500 to 700 nm,unless the film thickness is sufficiently increased. However, since thefirst oriented film (1) is expensive, it is costwise disadvantageous toincrease the thickness of the first oriented film (1), and in addition,increasing the film thickness tends to increase the difficulty to insureoptical homogeneity. Moreover, it is not necessarily easy, in terms offilm-forming technology and drawing technology, to manufacture the firstoriented film of substantial thickness. Therefore, by taking advantageof the addition property of retardation values R, the deficiency inretardation value R is compensated for by laminating (1) with the secondoriented film (2) and at the same time paying attention to therefractive index n_(D) of polymer film for the second oriented film (2),it is insured that no random reflection will occur between the firstoriented film (1) and second oriented film (2). It should be understoodthat two or more units of the optical compensator of the invention canbe assembled into a single liquid crystal display device.

Effects of Invention

Since the optical compensator of the present invention has both anecessary retardation value R and a necessary wavelength dispersionvalue ν_(RF) thanks to an ingenious combination of said first orientedfilm (1) and second oriented film (2), the liquid crystal displayincorporating this optical compensator is remarkably improved in thecoloration and contrast ratio which are the drawbacks of the FTN modewhile the advantages of the mode, namely its minimal weight andthickness as well as brightness, are exploited.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing an exemplary optical compensator ofthe present invention;

FIG. 2 is a schematic view showing the architecture of a liquid crystaldisplay device incorporating the optical compensator of the invention;

FIG. 3 is a schematic view showing the architecture of a liquid crystaldisplay device incorporating the optical compensator of the invention;

FIG. 4 is a schematic view showing the architecture of a liquid crystaldisplay incorporating the optical compensator of the invention;

FIG. 5 is a diagram showing the orientational relationship of axes inthe liquid crystal display used in the examples of the invention;

FIG. 6 is a diagram showing the orientational relationship of axes inthe liquid crystal display used in the examples of the invention.

The legends used on the drawings have the following meanings.

    ______________________________________                                        (1)        First oriented film                                                (2)        Second oriented film                                               (3)        Optically isotropic film                                           (4)        Pressure-sensitive adhesive layer                                  (5)        Release sheet                                                      (101)      Upper polarizer                                                    (102)      Liquid crystal cell                                                (103)      Substrate                                                          (104)      Transparent electrode                                              (105)      Oriented film                                                      (106)      Spacer                                                             (107)      Liquid crystal                                                     (108)      Lower polarizer                                                    (109)      Optical compensator                                                (201)      Upper polarizer                                                    (202)      Liquid crystal cell                                                (203)      Substrate                                                          (204)      Transparent electrode                                              (205)      Oriented film                                                      (206)      Spacer                                                             (207)      Liquid crystal                                                     (208)      Lower polarizer                                                    (209)      Optical compensator                                                (210)      Optical compensator                                                (301)      Upper polarizer                                                    (302)      Liquid crystal cell                                                (303)      Substrate                                                          (304)      Transparent electrode                                              (305)      Oriented film                                                      (306)      Spacer                                                             (307)      Liquid crystal                                                     (308)      Lower polarizer                                                    (309)      Optical compensator                                                (310)      Optical compensator                                                (401)      Angle of twist of liquid crystal                                   (402)      Direction of rubbing of upper substrate                            (403)      Angle from horizontal direction to                                            direction of rubbing of upper substrate                            (404)      Direction of rubbing of lower substrate                            (405)      Direction of polarization axis of upper                                       polarizer                                                          (406)      Angle from horizontal direction to                                            direction of polarization axis of upper                                       polarizer                                                          (407)      Direction of polarization axis of lower                                       polarizer                                                          (408)      Angle from horizontal direction to                                            direction of polarization axis of lower                                       polarizer                                                          (409)      Direction of orientation axis of optical                                      compensator                                                        (410)      Angle from horizontal direction to                                            direction of orientation axis of optical                                      compensator                                                        (501)      Angle of twist of liquid crystal                                   (502)      Direction of rubbing of upper substrate                            (503)      Angle from horizontal direction to                                            direction of rubbing of upper substrate                            (504)      Direction of rubbing of lower substrate                            (505)      Direction of polarization axis of upper                                       polarizer                                                          (506)      Angle from horizontal direction to                                            direction of polarization axis of upper                                       polarizer                                                          (507)      Direction of polarization axis of lower                                       polarizer                                                          (508)      Angle from horizontal direction to                                            direction of polarization axis of lower                                       polarizer                                                          (509)      Direction of orientation axis of upper                                        optical compensator                                                (510)      Angle from horizontal direction to                                            direction of orientation axis of upper                                        optical compensator                                                (511)      Direction of orientation axis of lower                                        optical compensator                                                (512)      Angle from horizontal direction to                                            direction of orientation axis of lower                                        optical compensator                                                ______________________________________                                    

BEST MODE FOR CARRYING OUT THE INVENTION

The following examples are further illustrative of the invention. In thefollowing disclosure, all parts are by weight.

EXAMPLE 1

FIG. 1 is a sectional view showing an example of the optical compensatorof the invention.

First oriented film (1)

A solution prepared by dissolving 25 parts of a brominated phenoxyetherresin with a bromine content of 52.9 weight % (Tohto Kasei Co., Ltd.,YPB-43C), the chemical formula of which is presented below (formula 1),in 75 parts of a 60:40 w/w mixture of cyclohexanone and dioxane was caston a polyester film base and partially dried until the solvent residuewas 5 weight %. The resulting film was exfoliated from the polyesterfilm and dried until there was no solvent residue. The film thusobtained had a thickness of 70 μm, a refractive index of n_(D) =1.64, anAbbe number of ν_(D) =24, a DSC glass transition temperature of T_(g)=149° C., a retardation value of R=2 nm, and a thermal deformationtemperature of 111° C. (JIS K6911).

This film was drawn 3-fold in one direction at a temperature of 155° C.and subjected to aging at the same temperature for 2 seconds, afterwhich both edges were trimmed off. The above procedure gave a firstoriented film (1) having a thickness of 42 μm, a retardation value of480 nm and a wavelength dispersion value of ν_(RF) =1.14.

Second oriented film (2)

A polycarbonate film was prepared by casting. This film had a thicknessof 80 μm, a refractive index of n_(D) =1 59, an Abbe number of ν_(D)=30.3, a glass transition temperature of Tg=135° C. and a retardationvalue of R=4 nm.

The above film was then drawn 1.5-fold in one direction at 168° C. andsubjected to aging at the same temperature for 10 seconds, after whichboth edges were trimmed off. The above procedure gave a second orientedfilm (2) having a thickness of 64 μm, a retardation value of R=98 nm anda wavelength dispersion value of ν_(RF) =1.09.

Optical compensator

The above first oriented film (1) and second oriented film (2) werelaminated with an acrylic adhesive in such a manner that the respectivedirections of orientation would be parallel. Then, optically isotropicfilms (3), (3) each comprising a 50 μm cellulose triacetate film werelaminated on respective sides of the above laminate film with anurethane adhesive to provide an optical compensator having a laminarstructure of (3)/(1)/(2)/(3). This optical compensator had a retardationvalue of R=578 nm and a wavelength dispersion value of ν_(RF) =1.13.

Optical compensator with release sheet

A release sheet (5) having a pressure-sensitive adhesive layer (4) wasprepared by applying a acrylic pressure-sensitive adhesive in athickness of 30 μm on a release-treated side of a 40 μm-thick polyesterfilm release sheet and this release sheet was laminated on either sideof the optical compensator prepared above. To put the opticalcompensator to use, the release sheet (5) only is peeled off and theremainder is bonded to the substrate. Using this optical compensator, aliquid crystal display device comprising the polarizer/liquid crystalcell/optical compensator/polarizer was fabricated. FIG. 2 is a schematicview showing a liquid crystal display incorporating the opticalcompensator of this example. The liquid crystal cell (102) contains aliquid crystal (107) in the space defined by juxtaposed substrates (103)each carrying transparent electrodes (104) and rubbed oriented films(105) with the interposition of spacer means (106). The opticalcompensator (109) is positioned below said liquid crystal cell (102) andthe assembly is sandwiched between the upper polarizer (101) and lowerpolarizer (108). FIG. 5 shows the orientational relationship of axes inthe liquid crystal display of FIG. 2 as viewed from top. The referencenumeral (401) stands for the angle of twist of the liquid crystal, (403)for the angle from the horizontal direction to the direction of rubbing(402) of the upper substrate of the liquid crystal cell, (404) for thedirection of rubbing of the lower substrate of the liquid crystal cell,(406) for the angle from the horizontal direction to the direction ofpolarization axis (405) of the upper polarizer, (408) for the angle fromthe horizontal direction to the direction of polarization axis of thelower polarizer, and (410) for the angle from the horizontal directionto the direction of orientation axis (409) of the optical compensator.The direction of angle is positive when it is clockwise. The product Andof refractive index anisotropy Δn of liquid crystal and cell thickness dwas 0.86 μm. The angle of twist (401) of liquid crystal was set at 240°clockwise from down to up, the angle (403) was set at 30°, the angle(406) at 65°, the angle (408) at 95°, and the angle (410) at 50°. Thepolarizer used here comprised a polyvinyl alcohol-iodine polarizing filmand a cellulose triacetate film bonded to either side thereof and had avisible light transmissivity of 42% and a polarization degree of 99%.The liquid crystal sealed in the liquid crystal cell was a nematicliquid crystal giving a wavelength dispersion value of ν_(LC) =1.14.This liquid crystal was a composition which can be represented by thefollowing chemical formula (2). ##STR1##

This liquid crystal display had been improved remarkably in colorationand contrast ratio and somewhat in brightness, too, thus beingsubstantially comparable to a liquid crystal display using ahue-compensating liquid crystal cell.

EXAMPLE 2

First oriented film (1)

A polyethylene naphthalate film obtained by melt-molding was provided.This film had a thickness of 80 μm, a refractive index of n_(D) =1.65,an Abbe number of ν_(D) =18 and a DSC glass transition temperature ofTg=113° C.

This film was then drawn 1.9-fold in one direction at a temperature of130° C. and subjected to 4 seconds of aging at the same temperature,after which both edges were trimmed off. The above procedure gave afirst oriented film (1) having a thickness of 60 μm, a retardation valueof R=420 nm and a wavelength dispersion value of ν_(RF) =1.18.

Second oriented film (2)

A polycarbonate film was prepared by the casting technique. This filmhad a thickness of 80 μm, a refractive index of n_(D) =1.58, an Abbenumber of ν_(D) =30.3, a glass transition temperature of Tg=135° C. anda retardation value of R=150 nm.

This film was then drawn 1.6-fold in one direction at 170° C. andsubjected to 8 seconds of aging at the same temperature, after whichboth edges were trimmed off. This procedure gave a second oriented filmhaving a thickness of 62 μm, a retardation value of R=150 nm and awavelength dispersion value of ν_(RF) =1.09.

Optical compensator

The above first oriented film (1) and second oriented film (2) werelaminated with an acrylic adhesive in such a manner that the directionsof orientation of the two films would be parallel. Then, a couple ofoptically isotropic films (3),(3) each comprising a 50 μm thickcellulose triacetate film were bonded to respective sides of the abovelaminate to fabricate an optical compensator having an architecture of(3)/(1)/(2)/(3). This optical compensator had a retardation value ofR=555 nm and a wavelength dispersion value of ν_(RF) =1.16.

Using the above optical compensator, a liquid crystal display wasfabricated in otherwise the same manner as Example 1. This liquidcrystal display had been remarkably improved in coloration and contrastratio and was substantially comparable to a liquid crystal displayincorporating a hue compensating LC cell.

EXAMPLE 3

The procedure of Example 1 was repeated except that chlorinatedphenoxyether resin was used in lieu of brominated phenoxyether resin.The result was as satisfactory as the result obtained in Example 1.

Comparative Example 1

A polycarbonate film was prepared by the casting technique. This filmhad a thickness of 170 μm, a refractive index of n_(D) =1.58, an Abbenumber of ν_(D) =30.3, a glass transition temperature of Tg=140° C. anda retardation value of R=7 nm.

This film was then drawn 2-fold in one direction at 170° C. andsubjected to 6 seconds of aging at 165° C., after which both edges weretrimmed off. The procedure gave an oriented film having a thickness of110 μm, a retardation value of R=570 nm and a wavelength dispersionvalue of ν_(RF) =1.09.

Then, a couple of optically isotropic films each comprising a 50 μmthick cellulose triacetate film were bonded to respective sides of theabove oriented film with an urethane adhesive to fabricate an opticalcompensator.

Using this optical compensator, a liquid crystal display was fabricatedas in Example 1. This liquid crystal display showed an intense bluecolor and had a low contrast ratio.

Comparative Example 2

The procedure of Comparative Example 1 was repeated using polymethylmethacrylate, polyvinyl alcohol, ethylene-vinyl alcohol copolymer,polyethylene terephthalate, polybutylene terephthalate, polyethylene,polypropylene, polyvinyl chloride, polystyrene, phenoxyether polymer andpolyacrylonitrile. Thus, each of these polymers was cast into a film andmonoaxially oriented with a draft ratio of 1.5 to 4 and a couple ofoptically isotropic films each comprising a cellulose triacetate filmwere laminated on respective sides of the oriented film to fabricate anoptical compensator. Using this optical compensator, a liquid crystaldisplay was constructed and evaluated. As a result, all the displaysshowed similar colorations and had low contrast ratios just as inComparative Example 1.

EXAMPLE 4

A polyethylene naphthalate film having a refractive index of n_(D)=1.65, an Abbe number of ν_(D) =18 and a DSC glass transitiontemperature of Tg=113° C. was drawn in one direction and aged to providea first oriented film (1a), (1b) having a wavelength dispersion value ofν_(RF) =1.18.

On the other hand, a polycarbonate film having a refractive index ofn_(D) =1.58, an Abbe number of ν_(D) 32 30.3 and a DSC glass transitiontemperature of 135° C. was drawn in one direction and aged to provide asecond oriented film (2a), (2b) having a wavelength dispersion value ofν_(RF) =1.09.

With the retardation value R of the above first oriented film (1a) beingset to R=320 nm and the retardation value R of the second oriented film(2a) to R=100 nm, these films were laminated with an acrylic adhesive insuch a manner that the respective directions of orientation would beparallel and a couple of optically isotropic films (3), (3) eachcomprising a 50 μm thick cellulose triacetate film were bonded torespective sides of the laminate with an urethane adhesive to provide anoptical compensator having a laminar structure of (3)/(1a)/(2)/(3). Thisoptical compensator had a retardation value of R=420 nm and a wavelengthdispersion value of ν_(RF) =1.16. Using two units of this opticalcompensator film, a liquid crystal display having a structure ofpolarizer/liquid crystal cell/optical compensator/opticalcompensator/polarizer was fabricated. FIG. 3 is a schematic view of thisliquid crystal display. The liquid crystal cell (202) contains a liquidcrystal (207) in the space defined by juxtaposed substrates (203) eachcarrying transparent electrodes (204) and rubbed oriented films (205)with the interposition of spacer means (206). Disposed below this liquidcrystal cell (202) are said optical compensators (209), (210) and theassembly is sandwiched between an upper polarizer (201) and a lowerpolarizer (208). The orientational relationship of axes in this liquidcrystal display as viewed from the top of FIG. 3 is shown in FIG. 6. Thereference numeral (501) stands for the angular dimension of twist of theliquid crystal, (503) for the angle from the horizontal direction to thedirection of rubbing (502) of the upper substrate of liquid crystal cell(202), (504) for the direction of rubbing of the lower substrate ofliquid crystal cell (202), (506) for the angle from the horizontaldirection to the direction of polarization axis (505) of the upperpolarizer, (508) for the angle from the horizontal direction to thedirection of polarization axis (507) of the lower polarizer, (510) forthe angle from the horizontal direction to the direction of orientationaxis (509) of the upper optical compensator, and (512) for the anglefrom the horizontal direction to the direction of orientation axis (511)of the lower optical compensator. The clockwise angle is positive. Theproduct Δn·d of refractive index anisotropy Δn of liquid crystal andcell thickness d was 0.86 μm. The angle of twist (501) of the liquidcrystal was set, in the clockwise direction from down to top, at 204°,the angle (503) at 30° C. the angle (506) at 0°, the angle (508) at 90°,the angle (510) at 70°, and the angle (512) at 30°. This liquid crystaldisplay had been much more improved in coloration and contrast ratio ascompared with the liquid crystal display of Example 2 and was fullycomparable to a liquid crystal display incorporating a hue compensatingliquid crystal cell.

With the retardation value of the above first oriented film (1a) beingset to R=300 nm and the retardation value of the second oriented film(2a) to R =100 nm, these films were laminated with an acrylic adhesivein such a manner that the respective directions of orientation would beparallel and a couple of optically isotropic films (3) each comprising a50 μm thick cellulose triacetate film were bounded to respective sidesof the above laminate with an urethane adhesive to provide an opticalcompensator having a laminar structure of (3)/(1a)/(2a)/(3). Thisoptical compensator had a retardation value of R=400 nm and a wavelengthdispersion value of ν_(RF) =1.16. Using two units of this opticalcompensator, a liquid crystal display having an architecture ofpolarizer/optical compensator/liquid crystal cell/opticalcompensator/polarizer was fabricated. FIG. 4 is a schematic illustrationof this liquid crystal display. The liquid crystal cell (302) contains aliquid crystal (307) in the space defined by juxtaposed substrates (303)each carrying a transparent electrode (304) and a rubbed oriented film(305) with the interposition of spacer means (306). The opticalcompensators (309) and (310) are disposed on respective sides of theabove liquid crystal cell (302) and the assembly is sandwiched betweenthe upper polarizer (301) and the lower polarizer (308). Theorientational relationship of axes in the liquid crystal display of FIG.4, viewed from up, is similar to that shown in FIG. 6. Here, too, theclockwise direction of angle is positive. The product Δnd of therefractive index anisotropy of liquid crystal Δn and cell thickness dwas set to 0.86 μm. The angle of twist (501) of the liquid crystal, inthe clockwise direction from down to top, was set at 240°, the angle(503) at 30°, the angle (506) at 80°, the angle (508) at 10°, the angle(510) at 110° and the angle (512) at 70°. This liquid crystal displayhad been much more improved in coloration and contrast ratio as comparedwith the CLD of Example 2 and was fully comparable to a liquid crystaldisplay incorporating a hue compensating liquid crystal cell.

Using the liquid crystal cell described in Example 1, the followingliquid crystal displays were fabricated.

1. Polarizer/(1a)/(2a)/(1b)/liquid crystal cell/polarizer

2. Polarizer/(1a)/(2a)/(1b)/(2b)/liquid crystal cell/polarizer

3. Polarizer/(1a)/(1b)/(2a)/(2b)/liquid crystal cell/polarizer

4. Polarizer/(1a)/(2a)/liquid crystal cell/(1b)/polarizer

5. Polarizer/(1a)/(2a)/liquid crystal cell/(1b)/(2b)/polarizer

6. Polarizer/(1a)/(2a)/liquid crystal cell/(2b)/(1b)/polarizer

7. Polarizer/(1a)/(2a)/(1b)/liquid crystal cell/(1a)/(2b)/(1b)/polarizer

8. Polarizer/(1a)/(2a)/(1b)/(2b)/liquid crystalcell/(1a)/(2a)/(1b)/(2b)/polarizer

In the above structures 1 through 8, the retardation values R of thefirst oriented film (1a), (1b) and second oriented film (2a), (2b) wereset according to the respective structures. These liquid crystaldisplays had been still more improved in coloration and contrast ratioas compared with the LCD of Example 2, being fully comparable to aliquid crystal display incorporating a color compensating liquid crystalcell.

INDUSTRIAL APPLICABILITY

The optical compensator of the present invention is particularly usefulfor optical compensation in an STN (supertwisted nematic) liquid crystaldisplay, there is to say on the FTN mode. In addition, it can be used insuch applications as goggle transparent and antiglare transparent parts,optical filters and so on.

We claim:
 1. An optical compensator comprising a laminate, wherein thelaminate comprises:(1) at least one oriented film of a first typeobtained by stretching, in at least one axis, a polymer film having arefractive index n_(D) ≧1.60, an Abbe number ν_(D) ≦30.0 and a glasstransition temperature Tg=60° to 160° C., and (2) at least one orientedfilm of a second type laminated to said oriented film of the first type,said oriented film of the second type being obtained by stretching, inat least one axis, a polymer film having a refractive index n_(D) <1.60,wherein the oriented film of the first type has at least one axis oforientation which is parallel to an axis of orientation of the orientedfilm of the second type.
 2. The optical compensator of claim 1, whereinthe laminate film has a retardation value R=60 to 1000 nm, and theoriented film of the first type has a wavelength dispersion value ν_(RF)of not less than 1.10, wherein ν_(RF) is defined by the followingequation: ##EQU1## wherein Δn is the refractive index of the orientedfilm of the first type and d is the thickness of the oriented film ofthe first type.
 3. The optical compensator of claim 1, wherein thelaminate has a structure represented by (1)/(2), wherein (1) is anoriented film of the first type and (2) is an oriented film of thesecond type.
 4. The optical compensator of claim 1, wherein the laminatehas a structure represented by (1)/(2)/(1), wherein (1) is an orientedfilm of the first type and (2) is an oriented film of the second type.5. The optical compensator of claim 1, wherein the laminate has astructure represented by (1)/(1)/(2), wherein (1) is an oriented film ofthe first type and (2) is an oriented film of the second type.
 6. Theoptical compensator of claim 1, wherein the laminate has a structurerepresented by (2)/(1)/(2), wherein (1) is an oriented film of the firsttype and (2) is an oriented film of the second type.
 7. The opticalcompensator of claim 1, wherein the laminate has a structure representedby (2)/(1)/(2)/(2), wherein (1) is an oriented film of the first typeand (2) is an oriented film of the second type.
 8. The opticalcompensator of claim 1, wherein the laminate has a structure representedby (1a)/(2a)/(1b), wherein (1a) is an oriented film of the first type,(1b) is an oriented film of the first type, and (2a) is an oriented filmof the second type.
 9. The optical compensator of claim 1, wherein thelaminate has a structure represented by (1a)/(2a)/(1b)/(2b), wherein(1a) is an oriented film of the first type, (1b) is an oriented film ofthe first type, (2a) is an oriented film of the second type, and (2b) isan oriented film of the second type.
 10. The optical compensator ofclaim 1, wherein the laminate has a structure represented by(1a)/(1b)/(2a)/(2b), wherein (1a) is an oriented film of the first type,(1b) is an oriented film of the first type, (2a) is an oriented film ofthe second type, and (2b) is an oriented film of the second type.
 11. Aliquid crystal display comprising:(a) a liquid crystal cell, (b) a pairof polarizers disposed on respective sides of said liquid crystal cell;and (c) at least one optical compensator interposed between said liquidcrystal cell and one polarizer of said pair of polarizers, said opticalcompensator comprising a laminate which comprises: (1) at least oneoriented film of a first type obtained by stretching, in at least oneaxis, a polymer film having a refractive index n_(D) ≧1.60, an Abbenumber ν_(D) ≦30.0 and a glass transition temperature Tg=60° to 160° C.,and (2) at least one oriented film of a second type laminated to saidoriented film of the first type, said oriented film of the second typebeing obtained by stretching, in at least one axis, a polymer filmhaving a refractive index n_(D) <1.60, wherein the oriented film of thefirst type has at least one axis of orientation which is parallel to anaxis of orientation of the oriented film of the second type.
 12. Theliquid crystal display of claim 11, wherein the laminate film has aretardation value R=60 to 1000 nm, and the oriented film of the firsttype has a wavelength dispersion value ν_(RF) of not less than 1.10,wherein ν_(RF) is defined by the following equation: ##EQU2## wherein Δnis the refractive index of the oriented film of the first type and d isthe thickness of the oriented film of the first type.
 13. The liquidcrystal display of claim 12, wherein the laminate has a structurerepresented by (1)/(2), wherein (1) is an oriented film of the firsttype and (2) is an oriented film of the second type.
 14. The liquidcrystal display of claim 12, wherein the laminate has a structurerepresented by (1)/(2)/(1), wherein (1) is an oriented film of the firsttype and (2) is an oriented film of the second type.
 15. The liquidcrystal display of claim 12, wherein the laminate has a structurerepresented by (1)/(1)/(2), wherein (1) is an oriented film of the firsttype and (2) is an oriented film of the second type.
 16. The liquidcrystal display of claim 12, wherein the laminate has a structurerepresented by (2)/(1)/(2), wherein (1) is an oriented film of the firsttype and (2) is an oriented film of the second type.
 17. The liquidcrystal display of claim 12, wherein the laminate has a structurerepresented by (2)/(1)/(2)/(2), wherein (1) is an oriented film of thefirst type and (2) is an oriented film of the second type.
 18. Theliquid crystal display of claim 12, wherein the laminate has a structurerepresented by (1a)/(2a)/(1b), wherein (1a) is an oriented film of thefirst type, (1b) is an oriented film of the first type, and (2a) is anoriented film of the second type.
 19. The liquid crystal display ofclaim 12, wherein the laminate has a structure represented by(1a)/(2a)/(1b)/(2b), wherein (1a) is an oriented film of the first type,(1b) is an oriented film of the first type, (2a) is an oriented film ofthe second type, and (2b) is an oriented film of the second type. 20.The liquid crystal display of claim 12, wherein the laminate has astructure represented by (1a)/(1b)/(2a)/(2b), wherein (1a) is anoriented film of the first type, (1b) is an oriented film of the firsttype, (2a) is an oriented film of the second type, and (2b) is anoriented film of the second type.